1. Field of the Invention
The invention relates to a method for manufacturing and converting of paper.
2. Description of the Related Art
During the paper manufacturing process a multitude of characteristics, mostly mechanical or technological paper characteristics which are important for one or more subsequent converting processes are determined. These change during production to a greater or lesser extent, even within a specific type of paper, and across the web width. A particular converting process may, for example, be a printing process, a formatting process, specifically a cutting process, etc.
Web shrinkage for example, is dependent not only on the specific cross width position, but also on the fiber characteristics and the tensions in the paper machine.
The aforementioned position related information regarding web characteristics are not available to the printer. As far as shrinkage is concerned, the printer is possibly familiar with the shrinkage across the entire width of an entire roll.
One of the most pressing problems in news print are the varying width expansion characteristics in the paper during printing, specifically during color printing. The reason for this is the effect of moisture on the paper during printing. During a multi-color printing process the web runs through several printing stations within a short time period. Hitherto the art work had to be adjusted to the expansion characteristics of the paper, resulting in corresponding time and raw material losses at the printers.
Individual paper rolls display individually different moisture related expansion characteristics. Paper, manufactured at the edge of the paper machine, displays a greater moisture expansion, as well as related greater tolerance values.
During the production process through to completion, the paper shrinks in varying degrees, in transverse direction to the web direction.
Two possible cross directional shrinkage curves are illustrated in FIGS. 1a and 1b. In each of the illustrations, the operator side shrinkage is depicted on the left, and the drive side shrinkage in the paper machine is depicted on the right.
The shrinkage is generally uniformly constant in the center, while the paper tends to increasingly shrink toward both edges. While the shrinkage depicted in FIG. 1a is constant in the center, this constancy is not achieved in certain instances (see FIG. 1b). A different shrinkage behavior may also occur due to various raw material combinations and production processes. As may also be seen in FIGS. 1a and 1b, the curve profile may differ greatly in the edge areas, whereby it is always non-linear in this area. Attempts are made through constructive measures to expand the center area as much as possible.
The paper produced in the paper machine is subsequently wound on so-called reel spools. The resulting fully wound rolls normally have a width of 2 m to more than 10 m and a diameter of almost 4 m, and possibly more.
Initial converting of these fully wound rolls takes place at the paper manufacturer. The preference is to convert paper for news print into smaller rolls on roll slitters. Common widths are 0.3 m to 1.4 m, sometimes even 2.8 m and diameters from 0.9 m to 1.5 m, whereby the tendency here too is for larger dimensions. Paper intended for cardboard packaging is usually cut into individual sheets on sheet cutters.
For ease of presentation we refer in the following text primarily to roll paper. However, the same conditions also apply to sheet paper.
During the printing process the paper expands transversely to the direction of travel. This expansion, or fan out, is in direct relationship to the shrinkage described above.
Paper, that shrank uniformly across its entire width during production, expands (fans out) again uniformly during printing. In partially multicolored printed paper, this fan out may be compensated for by offsetting the printing plates in the subsequent print stations by some 1/10 mm toward the web edge. Also, targeted expansion of the printing plates, for example in full-area color printing, is possible.
Paper that did not shrink uniformly during production becomes problematic with increasing roll widths. Therefore, non-uniform shrinking during paper production results in non-constant expansion or fan out during printing. Registration problems may only be compensated partially through the aforementioned measures. Fan out during printing may measure from several tenths of a millimeter to 3 mm or more, depending on the type of paper and the specific location on the roll. With increasing width of rolls that do not have constant transverse shrinkage, it becomes increasingly more difficult, particularly with full-area color print, to achieve the desired quality print result. In order to achieve the optimum print results the printing plates should for example be expanded at the edge of the roll by 3 mm, whereby the expansion should reduce to 0.3 mm toward the roll center, in order to remain at a constant 0.3 mm for the remaining roll half. A further problem exists in that an uneven web tension may occur in the printing machine when converting such rolls. To begin with, this is to be attributed to the non-linear longitudinal expansion behavior which, in turn, is dependent on the respective transverse shrinkage during production. This uneven web tension increases the risk of web breaks on one side and may lead to registration problems on the other side of the roll and, in extreme cases, may lead to wrinkle formation.
In a method for influencing the fan out in wet-offset rotary printing—known from German patent document no. DE 199 18 399 A1—a width or cross directional fan out of the paper web is determined by measuring one run of a paper web between a pull roll of a pre-tensioning device located prior to several printing cylinders, and an outfeed pull roll following the printing cylinders. The determined width of cross directional fan out is utilized for the drive control of at least one of the pull rolls, in order to achieve a constant width, or transverse fan out of the paper web. In this instance, the values utilized for the control are measured in the printing press area.